Insulating Flat Roof (and moisture)

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Mark-J
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Insulating Flat Roof (and moisture)

Post by Mark-J » Tue May 07, 2019 6:39 pm

I'm about to install Kingspan (TR26) insulation panels between joists under a flat roof shed. The panels will be propped up and covered with plywood boards. It's a 6m x 5m roof with fibreglass on the exterior.

I am going to leave 2" gaps around the edges, to help with ventilation to let everything breath and to help with air circulation.

I apologise for my attempt at a drawing :)

Image

Is there anything else I need to do to prevent my tools from rusting, is a 2" gap big enough?
Mark-J
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Re: Insulating Flat Roof (and moisture)

Post by Mark-J » Wed May 08, 2019 11:59 am

This pic has a gap above the Kingpsan and below it. Would this be better?

Image
Mark-J
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Re: Insulating Flat Roof (and moisture)

Post by Mark-J » Wed May 08, 2019 9:28 pm

Also, do I need some, breathable membrane with that setup?

The Kingspan is 100mm thick and the joists are 170mm deep so there's a gap above and below the Kingspan of 35mm (so about 1.4")
cotswold builders
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Re: Insulating Flat Roof (and moisture)

Post by cotswold builders » Fri May 10, 2019 4:57 pm

Its a cold roof construction, either ram it tight with insulation, no gaps, then vapour barrier and board.

If its vented through, leave 50mm air gap, insulation v/b and board.
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Re: Insulating Flat Roof (and moisture)

Post by Mark-J » Wed May 15, 2019 11:29 am

Thanks all for the advice. I've just painted the floor with resin and I'm about to start on insulating the roof. Which of the following three configurations should I go for?

Image

Also, is it still advisable to use a breathable membrane? I'll be inserting vents either side of the joists to facilitate airflow if it's important to the breathable membrane question.
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Re: Insulating Flat Roof (and moisture)

Post by cotswold builders » Wed May 15, 2019 5:50 pm

Construct as C then vapour barrier under joists before boarding
Mark-J
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Re: Insulating Flat Roof (and moisture)

Post by Mark-J » Wed May 15, 2019 9:47 pm

Thanks CB.

When you say under joists, do you mean, literally affix the v/b under the joists?

Do you mean like this:

Image

Or this:

Image
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